The investigation said that both pilots of an ITA Airways flight that departed from John F. Kennedy International Airport on April 30 were allegedly sleeping in the cockpit as their Airbus 330 flew 38,000 feet over France.
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While one pilot slept during his designated sleeping break, the airplane's captain fell asleep too, investigators said.
Air traffic controllers told investigators they lost contact with the plane for about 10 minutes. Amid fears of a terror incident, they prepared fighter jets to intercept the plane, but the pilots eventually responded.
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John Nance, an ABC News contributing aviation analyst, called this situation "extremely dangerous," especially if the pilots were unable to monitor weather conditions and the plane's fuel status.
"The plane can still fly on autopilot, but this is not smart or safe," he said.
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ITA Airways, previously known as Alitalia, said the captain claimed the radios stopped working, but investigators found "strong inconsistencies between the statements made by the captain and the outcome of the internal investigation," according to a statement.
The Italian airline said in a statement that the captain's behavior "was not consistent with the rules dictated by the company."
The plane still managed to land safely in Rome, and ITA Airways has since fired the captain.
In April, pilots at Southwest Airlines and Delta Air Lines pressed airlines to address pilot fatigue amid climbing demand for travel and staffing shortages.
"Fatigue, both acute and cumulative, has become Southwest Airlines' number-one safety threat," the Southwest Airlines Pilots Association, or SWAPA, told airline executives in a letter.